Terahertz technologies utilize electromagnetic radiation generally in the frequency range between 100 GHz and 10 THz (i.e, wavelengths of 3 mm to 30 μm, energies of 0.4 to 40 meV, or equivalent blackbody radiation temperatures of 5 K to 500 K). Many non-metallic materials that are visually opaque are partially transparent or exhibit molecular resonances in the terahertz region. In particular, water vapor and other small polar molecules have very strong rotational absorptions at terahertz frequencies. Therefore, terahertz technologies have many potential applications in diverse fields, including molecular spectroscopy, space and atmospheric sciences, plasma physics, biology, medical imaging, remote sensing, and communications.
Historically, there has been much interest in terahertz technologies for high-resolution (i.e., high Q) spectroscopy and remote sensing for space, planetary, and Earth science. For example, much of the interstellar medium radiates in the terahertz region, somewhat above the cosmic microwave background, enabling terahertz measurements to probe star formation and the early universe. Planetary atmospheres have background temperatures of tens to several hundred degrees Kelvin, enabling terahertz observation of extraterrestrial atmospheric conditions. Furthermore, thermal emission lines in the terahertz region from gases in the Earth's stratosphere and upper troposphere provide important indicators of ozone destruction, global warming, and pollution.
Terahertz signals can have a high bandwidth and are potentially useful for free-space communications. Therefore, terahertz technologies may be useful for space-based communications, such as satellite-to-satellite. However, limited atmospheric propagation, due to water and oxygen absorption, has discouraged the development of terahertz technologies for radar and terrestrial communications. Nonetheless, the technology may be attractive for relatively secure short-range communications, such as wireless communications in situations in which limited broadcast range is desirable.
A highly desirable application of terahertz technology is to do imaging of objects at useful standoff distances in real time for object or pattern recognition. Because terahertz irradiation does not involve the health and safety issues of ionizing radiation, such as are a concern with X-ray imaging, applications of terahertz technologies may include noninvasive tomographic imaging or spectroscopic characterization of biological materials. Because terahertz radiation is nondestructive and can penetrate non-metallic and non-polarizing external coverings (e.g., clothing, semiconductors, plastics, packaging materials), the technology may be useful in security screening for hidden explosives and concealed weapons. Finally, terahertz imaging may also be useful for industrial processes, such as package inspection and quality control.
However, beyond basic science, applications in the terahertz region are relatively undeveloped. Much progress is still required to provide field-deployable terahertz systems, especially for military, anti-terror, and biomedical imaging applications. Terahertz applications remain relatively undeveloped because the terahertz region lies between the traditional microwave and optical regions of the electromagnetic spectrum, where electronic or photonic technologies have been developed for these purposes. Terahertz applications have been hampered due to the historically poor performance of terahertz components lying in the “technological gap” between these traditional electronic and photonic domains.
In particular, the generation and detection of electromagnetic fields at terahertz frequencies has been difficult. To date, active terahertz generators have only demonstrated relatively low power capability. Traditional electronic solid state sources based on semiconductors roll-off at high frequencies. Tube sources are difficult to scale, due to the extremely high fields and current densities required. Therefore, frequency conversion techniques have typically been used to reach terahertz frequencies, including upconversion of millimeter waves using electronic or multiple harmonic techniques, or downconversion from the visible or near-IR using frequency mixing/switching or nonlinear optical processes. Recently, terahertz sources based on quantum cascade lasers (QCLs) have produced relatively high power in a compact size. Improvements in semiconductor materials systems suggest that a miniature terahertz QCL capable of generating a few milliwatts of output power at room temperature may soon be obtainable. Nonetheless, the weak radiation output from passive and active terahertz sources, the low photon energies of terahertz radiation, and high atmospheric attenuation due to molecular absorption (e.g., water vapor) frequently results in a weak received terahertz signal that may be difficult to distinguish from noise. Therefore, terahertz detection can also be difficult.
Current terahertz detectors include both direct and heterodyne detectors. Direct detectors generally directly convert the received power to a voltage or current that is proportional to the incoming power. They are characterized by responsivity (the ratio of the voltage output signal divided by the input signal power, in VAN) and noise-equivalent-power (NEP, the input signal power to the detector required to achieve a signal-to-noise ratio of unity after detection, in W/Hz1/2). Examples of direct detectors include rectifiers, bolometers, and pyroelectrics. A common direct detector uses antenna coupling to a small area Schottky diode that responds directly to the THz electric field. Another direct detector is the conventional bolometer that consists of a radiation absorbing material that is coupled to a sensitive temperature-dependent resistor.
For shorter wavelengths (i.e., frequencies above 1 THz), direct detectors generally have good responsivity and are sensitive to a broad band of frequencies. However, direct detectors generally provide no frequency discrimination, unless they are coupled with an external resonator or interferometer. Furthermore, they are sensitive to incoherent background noise and interference. Finally, direct directors are typically very slow, with 1 to 10 ms response times required to obtain an adequate signal-to-noise. Therefore, direct detectors have been used mainly for wideband applications, such as thermal imaging.
There has been a need for a selective and tunable narrowband detector for terahertz spectrum analysis and imaging. Most solid state devices have had difficulty in this regard, because the electron energy relaxation times in such devices are typically much longer than the period of terahertz oscillations and terahertz energies are smaller than typical thermal energies. Therefore, the terahertz electromagnetic wave is oscillating too fast for free carriers to respond.
Recently, the direct detection of terahertz radiation by two-dimensional (2D) plasma waves has been demonstrated in a double-quantum-well (DQW) field-effect transistor (FET) with a periodic grating gate. See X. G. Peralta et al., Appl. Phys. Lett. 81, 1627 (2002) and X. G. Peralta et al., Int. J. High Speed Elec. and Syst. 12(3), 925 (2002), which are incorporated herein by reference. Plasma waves in a gated 2D electron gas (2DEG) can have relaxation times much shorter than electron relaxation times or transit times, and their excitation is not linked to an electronic transition. Therefore, coherent charge density oscillations (plasmons) in a high-mobility 2DEG can be exploited to circumvent ordinary electronic limits on maximum operating frequency in conventional solid state devices based on electron drift. As a result, the response of the DQW FET can be fast. This speed increase arises from the fact that 2DEG plasmons have energy relaxation times of order 10−10 sec., roughly ten times faster than for uncorrelated electrons. Also, typical 2DEG densities from 1010 to 1012 cm−2 and device features of 1-10 μm yield plasmon frequencies in the 100 GHz-1 THz range, making plasmon devices attractive for terahertz applications. Furthermore, the ability to electrically tune the 2DEG charge density and hence the plasmon resonance via a gate voltage in the DQW FET enables the detection of specific, user-selected millimeter-wave to terahertz frequencies.
In FIG. 1 is shown the prior DQW FET 10 of Peralta et al. The DQW FET 10 was fabricated from a modulation doped GaAs/AlGaAs DQW heterostructure grown on a semi-insulating GaAs substrate 11 by molecular beam epitaxy. The two GaAs quantum wells (QWs) 12 and 13 were 20 nm wide and separated by a 7 nm AlGaAs barrier. The nominal electron densities in the QWs 12 and 13 were about 2×1011 cm−2. The 4.2 K mobility was about 1.7×106 cm2/Vs. The upper QW 12 was buried 404 nm below the surface of the device. A 2 mm×2 mm mesa was defined by chemical etching and ohmic contacts to both QWs 12 and 13 were formed by evaporating and annealing NiAuGe over the edge and side of the mesa forming the source S and the drain D. A 70 nm thick TiAu grating gate 14 (with no metallization between the grating fingers) was evaporated on the surface of the device with the fingers of the grating parallel to the ohmic contacts, perpendicular to the current flow. The grating period was 4 μm.
The grating modulates the electron density in the QWs 12 and 13 under the metallized part of the gate 14 when a voltage Vg is applied, selects wave vectors of the excited plasmon, and produces both normal and transverse THz electric fields. A voltage Vapp is applied to the drain D to establish a drain-source channel current IDS. The current IDS is kept nearly constant using a load resistor R having a resistance much higher than the device resistance. The incident RF radiation 15 having frequency fRF resonates at the standing plasmon resonance and its spatial frequencies under the grating metallization, producing a photocurrent and leading to a decrease in the electrical resistance of the channel between the source S and the drain D. The photoresponse can be measured as change in an output drain-source voltage VDS that depends on the gate voltage Vg and the frequency fRF of the incident terahertz radiation 15.
In FIG. 2 is shown the gate-bias-dependent photoresponse at T=25 K for different incident RF radiation frequencies fRF for a DQW FET 10 having the 4 μm grating period. The positions and strengths of the peaks in the photoresponse are controlled by both the voltage, Vg, applied to the gate 14 and the period of the grating gate. For 570 GHz, there is a resonance in the photoresponse around Vg=−1.6 V. The resonant peak moves to lower negative gate voltage, therefore higher electron density, as the frequency fRF of the incident radiation 15 is increased. At gate voltages more negative than −1.8 V, both QWs 12 and 13 are fully depleted under the metallic portion of the grating gate, and the channel is pinched off. With this grating period, the DQW FET 10 displays a measurement-limited resonant response from 0.57 to 0.66 THz, tunable by the applied grating gate voltage. The strength of the resonant photoresponse was observed to be maximal at temperatures between 25 to 40 K, decreasing at both lower and higher temperature.
While the DQW FET of Peralta et al. can be used as a selective and tunable direct detector for some applications, heterodyned detection is desirable for some terahertz applications. Particularly for weak signals, heterodyning can be used to coherently downconvert the terahertz signal to increase signal-to-noise by reducing bandwidth. The downconverted signal can then be post-amplified and processed using conventional microwave techniques. Heterodyne mixers beat the signal RF frequency against a known local oscillator (LO) frequency to generate an intermediate frequency (IF) difference signal that is tunable through the local oscillator frequency. Mixers are characterized by conversion gain (η, the ratio of the IF output power to the absorbed RF signal power, in dB), IF bandwidth (Hz), noise temperature (K), and LO power required. Mixers display good rejection of incoherent noise and interference. They are typically fast, with IF bandwidths of 0.1 to 10 GHz. Furthermore, narrowband detectors do not require additional frequency selective elements to analyze the spectrum of the incoming THz radiation as long as the received RF signal is within an IF bandwidth of the LO frequency. Therefore, heterodyne detectors have been used in narrow frequency band, high-resolution applications at lower terahertz frequencies, such as for molecular spectroscopy.
Common mixers are field-type devices that have a strong quadratic nonlinearity. For applications where the sensitivity of room-temperature detectors is adequate, Schottky diode mixers are preferred for downconversion. For high-sensitivity detection requiring cryogenic cooling, superconductor-insulator-superconductor (SIS) tunnel junction mixers have been used for sub-THz signals having energies below the superconducting bandgap. An alternative to the SIS mixer is the transition-edge or hot electron bolometer (HEB). HEB mixers are based on the heating of a superconducting microbridge with the THz radiation. The nonlinear I-V curve, necessary for mixing, results from the electron heating of the microbridge, which experiences a superconducting transition. Since the bridge resistance is dependent on the electron temperature, the device voltage is proportional to the THz power received. Furthermore, since the heated electrons have a very short relaxation time (e.g., less than 1 nanosecond), the HEB has a very low noise temperature and is capable of a high-speed operation, enabling heterodyning at signal frequencies up to several terahertz. However, a fast solid-state terahertz radiation mixer is still needed to enable coherent detection for terahertz applications requiring high resolution.